Most creators who will use a font will just use it directly without giving a fallback and even if they do provide one the result would be that people would contact the merchant (and sometimes us) when they deliver the bought item to another grid and it doesn't to look as they expect.

That's one of the problems when selling content that isn't self contained. you can't rely on a non-standard font or texture UUID to exist on every grid that content may be delivered to.

In fact, this has weird knock-on effects for the rest of the simulator session.

If a script in an object tries to draw text using the Cambria font, size 12 (and possibly others), subsequent calls to osGetDrawStringSize() will always return <0, 1, 0> - even in other objects, even if they're using a supported font. There may be other oddities that I've not noticed, but this broke the scripts that Dot and I were testing.

Rebooting the simulator fixes it, but as soon as a script tries to use Cambria again, the problem resurfaces.

Thanks for the bug report! I have found the problem: several fonts (including Cambria) had the wrong file permissions in the filesystem, so Linux didn't allow OpenSim to use them. I'm not sure how this happened: my guess is that it was when we switched our Linux variant from Amazon Linux to Ubuntu. Anyway, this will be fixed in the next release.

Working on updates for my boards with in-world writing, I saw that the only "handwriting font" we had does't work anymore in Kitely:
Zapf Chancery
was installed previously and worked but now it fails!
Most of the other fonts in Orens list of installed fonts still work.
Can this please be fixed?

However, right now I fixed it using a "hack", so it might disappear from time to time. In the next update I'll make the fix official. But anyway, for development purposes, it should mostly work now, and it will completely work after the next update.